Project Summary Heart Failure (HF) is a clinical syndrome that is the leading cause of mortality in the United States. It is characterized by the heart?s inability to pump blood sufficiently to meet the metabolic demands of the body. Pathological remodeling of the heart is a hallmark of HF and these gross changes in the structure of the heart further perpetuates the disease. Cardiac fibroblasts are the critical cell type that is responsible for maintaining the structural integrity of the heart. Stress conditions, such as a myocardial infarction, activate and induce the transdifferentiation of quiescent fibroblasts into synthetic and contractile myofibroblasts. While required for the initial healing response, sustained myofibroblast activation leads to over-secretion of extracellular matrix proteins which promotes fibrosis. Furthermore, de novo expression of ?-smooth muscle actin allows these cells to contract, disrupting coordinated cardiomyocyte contractility. These structural and mechanical changes to the heart impair cardiac function. G protein-coupled receptor (GPCR) kinases (GRKs) are important mediators of cardiovascular homeostasis through dampening of GPCR signaling. Recently, non-canonical activities of these GRKs have been elucidated. GRK5 has been demonstrated to translocate to the nucleus in cardiomyocytes, promoting hypertrophic gene transcription through activation of nuclear factor of activated T- cells (NFAT) and nuclear factor ?B (NF?B). Interestingly, these transcription factors are also involved in fibroblast activation and transdifferentiation. Our preliminary data strongly suggests a critical role for GRK5 in the activation and transdifferentiation of fibroblasts. The long-term research goal is to elucidate novel insight into the complexity of HF and associated cardiac remodeling as well as the role of GRK5 in this process. The specific aims are as follows: (1) To assess the role of GRK5 in fibroblast activation and transdifferentiation in vitro and (2) To assess the role of fibroblast GRK5 in cardiac fibrosis and HF progression after myocardial infarction in vivo. Specific aim 1 will be conducted using overexpression and knockout models of GRK5 in cardiac fibroblasts. These fibroblasts will be treated with Angiotensin II (AngII) and Transforming Growth Factor ? to induce differentiation. Specific aim 2 will utilize fibroblast specific GRK5 knockout mice which will undergo myocardial infarction surgery to mimic a clinically relevant disease. Accomplishment of these specific aims will provide novel understanding of fibroblast physiology and GRK5?s role in fibroblast activation/transdifferentiation. In terms of public health, targeting of cardiac fibroblasts provides a novel translational approach for the treatment of HF and cardiac fibrosis.